Technical Field
The present invention relates to a composite film and to its uses. More particularly, the present invention relates to a composite film composed of a polyolefin resin and a diazo copying paper in which the nature of the composite film is fully utilized.
Heretofore, as support sheets or substrates of so-called diazo copying papers coated with a photosensitive diazo compound, pulp papers made from wood pulp as base materials and thermoplastic stretched or oriented films, the surfaces of which have been physically or chemically treated are being used in large quantities. Among these support sheets, pulp paper is unsuitable for use in applications wherein strength, durability and dimensional stability are required. Therefore, as a support sheet for a secondary original drawing paper or copying papers for drawings which are used outdoors, a polyester oriented film is used for the purpose of satisfying the above mentioned requirements. However, a support sheet consisting of a polyester oriented film exhibits poorer writability and erasability and is more expensive, as compared with a pulp paper. Therefore, a polyester support sheet is not always satisfactory.
Oriented films made from thermoplastic resins with finely divided inorganic fillers incorporated thereinto are also known. A polyolefin oriented film containing clay is used as a support sheet for diazo copying papers.
Thermoplastic resin films containing inorganic fillers have excellent writability, erasability, stiffness, feel, optical properties (e.g. hue, whiteness, luster, opaqueness), dimensional stability and durability, and thus is suitable for use as a support sheet for a diazo copying paper. Particularly, when calcium carbonate is used as the filler, the resulting thermoplastic resin film exhibits a high degree of whiteness and excellent light resistance, results in decreased wear of machines used for cutting, printing, etc., and is inexpensive. A monolayer film or multi-layer film made from a polyolefin resin containing calcium carbonate has excellent properties as paper and thus is considered to be useful as a support sheet for a diazo copying paper. Calcium carbonate possesses an excellent covering property and is capable of promoting ink drying and may also be used as a useful filler for general-purpose synthetic papers.
As a result of our various studies, however, we have found that when a calcium carbonate-filled polyolefin resin film is used as the support sheet for a diazo copying paper, the diazo compound undergoes denaturation during storage of the paper, which results in fogging in which the portion of the paper to be originally whitened becomes colored upon being developed. Particularly, when the copying paper is stored under high temperature and humidity, this phenomenon becomes pronounced.
Results of our investigations indicated that the fogging is ascribable to the fact that the calcium carbonate-filled polyolefin resin film is basic or alkaline in nature. That is, the photosensitive diazo paper undergoes development of a color due to a coupling reaction under alkaline conditions. The calcium carbonate-filled polyolefin resin film has a pH of from about 9 to 10, measured by immersing 1 g of fine pieces of the film in 75 ml of distilled water at a temperature of 20.degree. C., stirring the mixture for about one hour, and determining the pH of the aqueous phase. This pH level is high enough to cause the fogging of the photosensitive diazo compound.
As an approach to preventing the fogging due to the calcium carbonate filler, mention may be made of the application of an acidic liquid on the film surface, the incorporation of a large amount of an acidic component into the photosensitive diazo compound, and lamination of a resin film on the film.
However, application or incorporation of an acidic material causes the development time of the diazo paper to be extended and, thus, is impracticable. On the other hand, when lamination is attempted, if a film made from a non-polar resin is laminated with a sufficient thickness on the film so as to isolate the calcium carbonate from the photosensitive diazo compound, the development of fogging may be completely prevented. However, the above mentioned preferable surface properties (writability, erasability, adhesion to the diazo photosensitive layer, etc.) are undesirably lost.